Détails:The flight Florence-Roma was to make an unscheduled stop at Pisa to fly some passengers there who had been stranded the day before due to a mechanical defect of their plane.The pilot chose to fly the short Florence-Pisa leg keeping the aircraft below the cloud base. Presumably the weather got suddenly worse, and the plane entered the clouds at low level. The pilot drifted briefly from the route and the plane crashed into a hill.